The present invention relates to a neon tube lighting device which lights a neon tube by means of a high-frequency, high-voltage power supply.
A conventional neon tube lighting device of this kind has a circuit arrangement such as shown in FIG. 1. The AC output of a commercial power source 11 is rectified by a full-wave rectifier 12, the rectified output from which is smoothed by a smoothing circuit 13, the output from which is, in turn, provided to a series circuit of transistors 14 and 15 and a series circuit of capacitors 16 and 17. A primary winding 19 of a transformer 18 is connected between the connection point of the transistors 14 and 15 and the connection point of the capacitors 16 and 17, a neon tube 22 is connected across a secondary winding 21 of the transformer 18, and both ends of a tertiary winding 23 of the transformer 18 are connected to the bases of the transistors 14 and 15, respectively, thus constituting a feedback circuit. The transistors 14 and 15, the capacitors 16 and 17, and the windings 19 and 23 make up a self-excited oscillator. The oscillation frequency of this oscillator is 9.5 kHz, for instance. The magnetic circuit of the transformer 18 constitutes a closed magnetic circuit.
In the conventional neon tube lighting device depicted in FIG. 1, shorting of a load, i.e. the neon tube 22 reduces the impedance of the transformer 18 to zero and an excessive current flows through the transistors 14 and 15, breaking them down. To prevent this, some protective circuit is needed. The total load changes with the length of the neon tube 22 and the number of such tubes connected, and the power source current also changes to vary the brightness of the neon tube 22. With such a load variation, the oscillation frequency of the oscillator is liable to vary since it is a self-excited oscillator. Even if it is provided with a constant-current characteristic by use of a leakage transformer as the transformer 18, the constant-current characteristic itself varies.
Furthermore, the neon tube lighting device shown in FIG. 1 is defective in that the neon lamp lacks stability in discharge. Especially, a decrease in the tube diameter of the lamp and an increase in its current density will both lead to the generation of an irregular discharge and what is called a stripe pattern. When the tube current is 15 mA, a neon tube 15 mm in diameter does not produce the stripe pattern, but a neon tube of a 6 mm diameter produces it; when the tube current is 30 mA, both tubes generate the stripe pattern.